Selecting an Oil for High Pressure Hydrogen Compressor

[Q_Goest]

I'm in the process of designing a high pressure hydrogen compressor. In this machine, an oil will be in direct contact with the hydrogen. To minimize the amount of oil vapor that enters the hydrogen and must be separated, an oil with high molecular weight and low vapor pressure should be best.

The quantity of oil vapour in the compressor depends to a large degree on the molecular distribution of the oil.

In a gas-chromatographic analysis, mineral oil based products show a typical broad, bell-shaped distribution of the molecular weight with a high proportion of short molecular weights, whereas PAOs have a pointed curve, in which the individually mixed base oil portions can be seen.

The major difference is that PAO contains very few short-chain molecules compared to the mineral oil.
It is these short-chain molecules that evaporate most easily.

. . .

In brief, the oil vapour content of synthetic oils is four to five times lower than same-viscosity mineral oils.
And, looking at trends it can be said that the higher the viscosity of the compressor oil, the lower the oil vapour content.

Ref: http://www.engineeringtalk.com/news/klu/klu131.html

The oil used in my compressor doesn't need to be a particularly good lubricant, it is used to displace hydrogen and compress it. My thinking is to use a vacuum pump oil instead of a conventional compressor lubricant. The vacuum pump oils have a vapor pressure on the order of 0.0001 micron. They can achieve this by distilling down the oil and removing the lower molecular weight parts.

Edit (this part added): Another concern regards how much hydrogen might disolve into the oil. Does anyone know if hydrogen disolves readily in mineral oil?

I haven't looked into compressor oil enough yet to know how they compare. Any comments or suggestions?

 

[FredGarvin]

I came across this doing a search on the Ostwald coefficient and hydrogen:

http://aiche.confex.com/aiche/2005/preliminaryprogram/abstract_21861.htm

It's not a detailed explanation, but it does give a couple of sources for research if needs be.

I will look at our vacuum pump facility and see what we use. Most of the time I can call and ask the manufacturer directly. Let me see what I can dig up. Hydrogen solubility is not exactly something I get to deal with...ever.

No chance in going with an oil free compressor since you need it for the compression, eh?

 

[Q_Goest]

Thanks Fred. Sounds from your reference that they're implying hydrogen disolves readily in hydrocarbon oil, something which I was afraid of. Looks like I'll have to either have it measured or find someone that can calculate it though I doubt I'll find anyone. Fortunately I have an easy option to have it measured, so I just may do that.

I tried talking to the company rep for the oil, but they didn't seem to know much. Also, because of the way the compressor is designed I can't design out the oil/hydrogen interface very easily. Maybe if I start all over ...

 

[sid_galt]

Hi Q_Goest,

Not sure if this will help but here's a paper discussing the solubility of hydrogen and other gases in alkanes and 1-alkenes

http://www.owlnet.rice.edu/~aghosh/gas_solubility.pdf

Best of Luck!

 

[Q_Goest]

Thanks Sid. I'll have a look.